This invention relates generally to techniques for measuring the frequency response characteristics of a device such as a bandpass filter, for an example of a passive device, particularly those applications where low level signals are required to be detected and measured.
A principal application of frequency response measurements is with devices having normal use in handling signals over some bandwidth wherein it is desired to determine exactly what the frequency response characteristic of the device is. A known alternating current signal is applied to the device and its response is measured as a function of frequency of the applied test signal. The simplist means of carrying out such a measurement is to use a varying frequency output signal generator applying a controlled frequency signal to the device with a voltmeter measuring its response. Besides passive devices, active devices such as amplifiers may be tested in this way. The frequency output of the signal generator is varied and the response of the device noted by reading the voltmeter level for each of the different frequencies applied to the device as adjusted on the signal generator.
For faster testing, a swept frequency signal generator is used which repetitively generates an output signal transversing a frequency range from a first frequency to a second frequency during which the response of the device under test is measured, with a return time existing between each cycle wherein the signal generator resets to its first frequency. The output of the device under test is then displayed on an oscilloscope whose horizontal sweep is driven by the same signal generator, the retrace of the sweep occurring during the period that the signal generator resets itself from the second frequency back to the initial first frequency. If a device is being tested with high frequencies, such as in the radio frequency range of the spectrum, a detector is generally utilized at the output of the device under test convert the high frequency signal to be measured into a direct current signal level which varies in proportion to the amplitude of the high frequency signal from the device under test. If low level signals are to be measured, a low frequency amplifier is then utilized at the output of the detector to raise its output signal level to a magnitude sufficient to present a satisfactory cathode ray tube display. It is in conducting tests with low level signals wherein inaccuracies result from any non-linearities in the detector/amplifier circuitry and noise that may exist in the circuit under test that is of a significant level when compared to the low level signals being measured. A signal generator power output can be made higher to over come the signal-to-noise ratio problems in a device under test but this is undesirable when the signal level exceeds that for which the device is designed to work in actual practice since it may be overdriven or otherwise change its characteristics because of the large signal. Furthermore, signal generators having large power outputs are extremely expensive.
The detector normally utilized is of a broadband type; that is, it accepts and detects any frequency within the range of the signal generator without any tuning. A different type of detector that is sometimes utilized, but which is generally undesirable because of its complexity, high cost and slow response time, is a narrow band detector utilizing a frequency selective receiver that is tunable in synchronism with the signal generator. Such a detector has the advantage, however, of significantly suppressing noise and interferring signals outside the test frequency range of interest while a broadband detector will not so discriminate. The most common broadband detector elements are semiconductor diodes, thermistors or barretters. The most usual noise and interferring signals include direct current and power line induced frequencies of 60 Hz and several harmonics thereof. The term "noise" as used herein is meant to include these undesired signals.
The sensitivity of existing measuring circuits utilizing a broadband detector followed by a high gain, low frequency amplifier results principally from undesired noise present in the output of the device under test which is large compared with the signal to be measured and displayed. The noise is usually within the bandwidth of a detector and amplifier. Non-linearity of the detector response as a function of frequency, and noise contributions and drift of the high gain amplifier, also contribute to a limit on the sensitivity of such devices.
One approach to overcome these difficulties and increase the sensitivity of such measuring devices has been to include capacitive coupling in each of two leads of the detector input to isolate the detector from direct current and low frequency noise of the device under test. However, such devices utilize the detector input capacitors as part of the input loading impedance of the device under test. The reactances of these capacitors are not optimized to maximize the signal-to-noise ratio so long as they must be selected for proper loading of the device under test. Proper impedance matching with the device under test becomes more critical as the test frequency increases because of the generation of standing waves. Also, the value of the capacitive isolation of the detector is not fully realized because of the grounding that is used in the detector and high gain amplifier circuit.
Another approach to increase the sensitivity of such measuring instruments that is presently commercially available is to modulate the signal generator output at a frequency somewhere inbetween the frequency of the undesired noise and the signal detector output frequency. After detection and amplification of the signal received back from the device under test, this signal is synchronously demodulated and then applied to the cathode ray tube for display. This technique has the advantage of rejecting undesired noise and minimizing errors from drift of the detector amplifier circuits and noise introduced therein. However, this technique has a serious disadvantage that the device under test may not respond in the same way to the modulated signal generator output as it does to a continuous signal.
Another, but related, technique that is presently available in commercial instruments is to apply alternating current modulation and demodulation around only the high gain amplifier which removes the principal problem of the technique described in the prior paragraph since the device itself is not subjected to a modulated waveform. But this technique suffers from the disadvantage that such a circuit becomes susceptible to the interferring signals present at the amplifier input. Both this and the technique described in the prior paragraph suffer from a further disadvantage that they are slow in response, and further both techniques are very expensive to commercially implement.
Therefore, it is a principal object of the present invention to provide a method and circuit implementation thereof for improving the sensitivity of a detector/amplifier combination in device testing apparatus without the aforementioned disadvantages that now exist with present techniques. It is also desired to do this in a manner that is less complex and less expensive than the techniques of the prior art described above.
It is a further object of the present invention to provide an improved technique for compensating for non-linear frequency varying characteristics of broadband detectors and for correcting for drift in high gain low frequency amplifiers.
It is yet another object of the present invention to provide an improved technique of superimposing various frequency markers on a signal displayed on a cathode ray tube for more rapid and sure identification of the frequency points along the display.